Letting light into a house is an important function of windows. Natural light is important to the health and well-being of the occupant. Even
though people have become more reliant on electric light in their
houses, good home design can provide most, if not all, the needed
daytime light. The introduction of natural light is also a powerful
architectural tool in shaping and defining the interior spaces. However,
providing daylight requires thoughtful window placement and interior
design that addresses a number of concerns such as visual comfort,
balanced light levels, color, and fading of furnishings.

Guidelines for Providing Natural Light

Arrange windows to provide daylight to all occupied rooms.

Locate windows to define and enhance architectural volumes.

Provide balanced lighting by introducing daylight from two directions in order to avoid glare and bright visual hot spots.

Place windows so that direct sunlight, if admitted at all, reflects off interior walls and floors to provide more diffuse, even light.

Use reflective ground surfaces or walls to increase daylight distribution into south- and north-facing windows.

Use translucent glazings on skylights to diffuse direct sunlight, if necessary.

Consider installing shutters or shades to block high midday summer sun while admitting daylight in morning, early evening, and on overcast days.

Use tubular daylighting devices (TDDs), also known as tubular skylights or light pipes, to provide bright, diffused sunlight in a particular room location.

Use landscape elements to block low direct sun into east- and west-facing windows.

Use shades/curtains/overhangs to block direct sunlight.

Use light from the north-facing windows to provide less variable, more diffuse illumination when desired.

Providing Balanced Lighting
A balance of light is important both for visual comfort and to perform
visual tasks. Too much contrast between dark and light, from very bright
light to very dark shadows, can be uncomfortable for the eyes. Although
the eyes can adjust to changes in light levels very quickly and can
simultaneously see a wide range of light intensities, the human eye is
more comfortable with a ratio of the brightest to the darkest level of
no more than twenty to one. Light levels for reading in the home or
office range from 100–2000 lux, and an overcast sky can provide
outdoor levels of 5000–20,000 lux. A beam of bright sunlight will
provide up to 100,000 lux on a surface. In order for direct sunlight to
be useful visually, it should be diffused and reflected around the room. When the sunlight is spread out over a larger area, it
provides more comfortable light levels.

It is important to recognize that different uses of interior spaces have
different ranges of acceptable lighting level. In a corridor, the amount
of illumination can range above or below desired levels with little
adverse impact. However, in a home office with a computer screen, visual
comfort depends on careful control of brightness ratios. Artwork and
artifacts, particularly those with paints, dyes, paper, or fabric that
are light-sensitive or susceptible to fading, must also be protected
from excessive light levels.

The balance of light in a space depends on the overall number and size
of windows, their location, and the average reflectance of the interior
surfaces of a room. A room with only one window will inevitably have
bright areas close to the window and dark areas farther from the window.
This gradation in light will be further exaggerated if the room has dark
surfaces and furnishings. An improved balance of light can be created by
providing light from at least two directions, such as windows located on
different walls or a skylight balancing the light from a window. Shadows
created from the first window source are balanced by light from the
second direction, and the overall contrast is reduced.

As the sun moves through the sky during the day and in different seasons
of the year, its angle of penetration into a room changes significantly.
In the middle of the day during the summer in the northern hemisphere,
for example, the sun comes in high overhead and strikes near the sill of
a south-facing window. However, the low sun to the east in early
morning and to the west during the afternoon will penetrate deep into a
room, strike back walls, and may shine directly into people's eyes. In the
winter, the sun is lower in the sky throughout the day for all
orientations, and tends to penetrate more deeply into rooms. This may
provide useful heat but must be managed to control glare. In order to be
visually useful, direct sunlight must first be reflected off of a floor
or wall and then diffused around the room. For instance, east- or
west-facing windows can be placed near a corner with a north wall of a
room, which will catch and reflect the sunlight before it penetrates too
deeply. In addition to designing the room itself properly for glare
control, venetian blinds, translucent shades, and drapes can be used to
diffuse entering light. In hot climates, it is usually best to avoid
direct sun completely during the peak cooling season.

Using Reflective Outdoor Surfaces to Increase Daylight
Direct light from the sun and sky is an important source of daylight,
although in hot climates, it is usually best to avoid direct sunlight
completely during the cooling season. In all climates there are
situations in which the light reflecting off of outside surfaces can be
very useful. A view of a light-colored sunlit wall can actually provide
more light than a view of the north sky. And large reflective horizontal
surfaces outside a window, such as snow, a lake, or a sandy beach,
greatly increase the amount of light entering the window.

People often think of south windows as the sunny windows; however, the
view from a south window might look out at the dark, shady "back" of a
building, which is in stark contrast to the bright, sunlit foreground.
On the other hand, a north window can look out onto a brightly sunlit
wall or garden, providing both a great deal of reflected light and a
cheerful view of sunlit flowers or surfaces.

Avoiding Glare from the Sun
While sun penetration at south windows in winter can create some glare
problems, east- and especially west-facing windows are usually the
greatest offenders year round. People inevitably orient windows toward
the most interesting view. But when that view includes a reflective
surface such as snow, water, or sand, especially if the window faces the
east or west, the penetrating low sun problem is intensified. These
windows create the most difficult situations for solar control. While
the view may be highly desired, the glare and excess heat of the sun and
its reflections are not. Typically, these windows require active
operation of shades or blinds by the homeowner, and, until recently, the
only permanent solar control technology was tinted or reflective
glazing, plastic films applied to the glass, or screens.

Direct sun at low angles can be blocked by trees, shrubs, or garden
walls. Such landscape elements can be strategically placed to reduce
glare as well as heat gain through east- and west-facing windows. The
most important time to block low-angled sun is in the summer when the
sun rises and sets farther to the north of direct east and west, so
plantings should be located to account for this pattern.

Diffusing Direct Sunlight
Overhangs and other solid external architectural elements can block
direct sunlight completely for some time periods, while more open
elements such as lattice structures diffuse the daylight before it
enters the windows. This can assist in reducing
glare from the direct sun or sky, and can illuminate spaces with a
larger and more diffuse light source. Dark-colored woven fiberglass or
perforated metal screens mounted on the exterior of windows can also
reduce glare while still maintaining a high degree of visibility to the
outside on sunny days. Light-colored screens diffuse the transmitted
light but do not allow as much view to the outside.

Interior shades, drapes, or blinds can block or diffuse direct sunlight.
The ideal drapery to reduce glare from bright windows, while still
allowing a clear view out, is a loosely woven dark-colored drape. A
loosely woven light-colored drape will diffuse the daylight from the
window about the room and provide maximum privacy. However,
light-colored drapes will appear very bright in direct sunlight and it
may be difficult to see through them. Roll-up shades can be translucent,
allowing some diffuse light to enter, or opaque, which block all light.
Shades can also be made from woven screen materials in varying densities
and colors, producing a range of light control and visibility.
Reflective and tinted plastic roll-up shades reduce the sun's intensity
and provide a clear view out, but do not diffuse or scatter the incoming
light. When adjusted to the correct angle, horizontal venetian blinds or
miniblinds can block direct sunlight but permit diffuse light to enter
the room and often provide views out. Vertical blinds serve a similar
function and are particularly useful for controlling low-angle sunlight
at east- and west-facing windows.

Translucent glazing materials, such as frosted or patterned glass, can
diffuse sunlight very evenly. Glass block and translucent fiberglass
panels also provide light diffusion and visual privacy. The use of
translucent glazings for skylights and clerestory windows, which are not
in the visual field, is an excellent way to diffuse sunlight evenly
throughout a space. When used on windows at eye level, however, the
materials may become too bright for visual comfort. A frosted window in
the low afternoon sunlight will seem to glow with the intensity of a
searchlight. Because of this effect, clear glass with additional
interior shading devices generally allows for better control of the
light from sunlit windows.

Using Skylights
Skylights and roof windows can provide high levels of daylight, but view
is not a concern as it is with conventional windows. The direct light
from above can be diffused by using frosted glazing, by using a light
well where light-colored vertical surfaces just beneath the skylight
reflect and diffuse sunlight, or by using interior shades or blinds
operated from below. A single skylight is far more effective at lighting
a larger space on a sunny day if the sunlight is diffused either by the
glazing or the light well surfaces. The size, shape, and color of light
well surfaces influences their ability to diffuse and distribute light.

When a skylight and light well are difficult to incorporate into the
room design, light tubes are becoming a popular approach to admit
sunlight into a room. A clear plastic dome (normally 10–14 inches in
diameter) is mounted over a hole in the roof and is connected to a small
light diffuser in the ceiling by a highly reflective tube. Sunlight
enters through the domed opening and bounces numerous times until it
emerges at the ceiling diffuser. Although some of the
light is lost in the multiple reflections in the tube, these systems can
provide as much light as a bright lighting fixture on a sunny day. The
overall efficiency of the system depends on the time of day and location
of the domed opening as well as the length of the tube and its internal
reflectivity.

The American Architectural Manufacturers Association's (AAMA) Skylight Council has a fact sheet, "Daylighting Basics: Daylighting and Energy Savings" that provides useful information on top lighting and its benefits. The document is available to download from AAMA's web site.

Using Light from the Sky
Light from the sky, as distinguished from light from the sun, is cooler,
gentler, and diffuse. While sunlight is normally considered to appear
white, the early morning and late afternoon sun takes on a yellow or red
hue, an effect that can be heightened by dust and pollutants in the
atmosphere. Depending upon the position of the sun and type of clouds
present, light from the sky will provide illumination levels of 5000–20,000 lux, or 5–20% of that provided by bright direct
sunlight. Light from a clear blue sky has the additional advantage that
there is more visible light, with a significantly smaller infrared
component.

In situations where daylight is desired with minimal solar heat gain,
north windows can provide the best quality of daylight of any
orientation. While the use of
north-facing glass may be desirable in terms of daylighting and avoiding
solar heat gain, it is not an effective strategy for providing useful
solar gain in winter.

Interior Strategies
You can increase the levels of natural daylight in your home by the use of certain interior design strategies. You can use interior windows to bring natural light deeper into spaces. These windows can be clear or translucent, large or small, operable or fixed, placed in various places on a wall surface — all part of the design of the space.

You can also increase the light levels in your home by the choice of interior surface materials. You can use materials such as light-colored or reflective paints or wallpaper, mirrors, metallic, or shiny surfaces that reflect or redirect light.